Due to its characteristics of being easily applicable to various products and electrical properties such as a high energy density, a secondary battery is not only commonly applied to a portable device, but universally applied to an electric vehicle (EV) or a hybrid electric vehicle (HEV) and an energy storage system that is propelled by an electric motor. This secondary battery is gaining attention for its primary advantage of remarkably reducing the use of fossil fuels and not generating by-products from the use of energy, making it a new eco-friendly and energy efficient source of energy.
A battery pack for use in electric vehicles has a structure consisting of a plurality of cell assemblies connected in series, each cell assembly including a plurality of unit cells, to obtain high power. Also, the unit cell includes a positive electrode current collector and a negative electrode current collector, a separator, an active material, and an electrolyte solution, and allows repeated charging and discharging by electrochemical reactions between the constituent elements.
In addition to this basic structure, the battery pack further includes a battery management system (BMS) to monitor and control a state of a secondary battery by applying an algorithm for control of power supply to a driving load such as a motor, measurement of electrical characteristic values such as current or voltage, charge/discharge control, voltage equalization control, and state of charge (SOC) estimation.
Recently, with the growing need for a high-capacity structure as well as utilization as an energy storage source, there is an increase in demand for a battery pack of a multi-module structure in which a plurality of battery modules including a plurality of secondary batteries connected in series and/or in parallel are assembled.
Because a battery pack of a multi-module structure is designed to have a plurality of secondary batteries arranged with a high density in a narrow space, it is important to easily discharge heat generated from the respective secondary batteries. One of the various methods of discharging heat generated from a secondary battery, an indirect cooling method is disclosed in Japanese Patent Application Publication No. 2009-301877.
FIG. 1 is a schematic exploded perspective view illustrating the design of a battery pack including a cooling pin and a secondary battery.
The indirect cooling method is a cooling method using a refrigerant through the cooling pin 2 in contact with a casing of the secondary battery 1. Heat generated from the secondary battery 1 is transferred through the cooling pin 2, and the cooling pin 2 transfers the heat to the refrigerant again. Generally, a heat sink 3 is formed on the top or bottom of the cooling pin 2 for quick heat exchange with the refrigerant.
A cooling method using a refrigerant as described above needs a separate space such as a passage along which a refrigerant moves. Also, the cooling method using a refrigerant needs a separate device such as a pump for moving a refrigerant. Thus, when a battery pack employs a cooling method using a refrigerant, not only a space for a secondary battery but also an additional space is necessary, and because an additional cooling system is mounted, an overall size of the battery pack increases and an energy density per unit volume of the battery pack reduces. Also, complexity of a cooling system results in an increase in failure factor such as a coolant leakage, clogging of a channel in which a refrigerant flows, and a failure in a pump or a motor for moving a refrigerant. Furthermore, due to having to mount an additional cooling system, an additional cost as well as a cost for fabricating a secondary battery is required. As a result, the price of the battery pack increases.